Environmental and health risks request removing harmful nitrogen oxides (NOX═NO and NO2) from exhaust, flue and off gasses to avoid them being released into the environment. The primary source of NOX is thermal formation when nitrogen and oxygen reacts at higher temperatures. During combustion processes where oxygen from the air is used, NOX is an unavoidable by-product and present in the exhaust gas generated from internal combustion engines, power plants, gas turbines, gas engines and the like. The release of NOx is typically regulated by legislation that is becoming increasingly more stringent in most areas around the world. An efficient method to remove NOX from exhaust or flue gasses is by selective catalytic reduction where the NOX is selectively reduced using ammonia (NH3-SCR), or a precursor thereof, as reducing agent (see Reaction 1-3). Selective catalytic reduction (SCR) of NOX by a reducing agent is an efficient way of reducing the amount of NOX in an exhaust, gas stream or flue gas. Typically, the reducing agent is a nitrogenous compound, such as ammonia or urea. For selective catalytic reduction using ammonia (NH3-SCR) desirable reactions include:4NO+4NH3+O2→4N2+6H2O  (Reaction 1)2NO+2NO2+4NH3→4N2+6H2O  (Reaction 2)6NO2+8NH3→7N2+12H2O  (Reaction 3)
In addition to SCR reactions several unwanted side-reactions can occur. A known issue is the unselective oxidation of ammonia that can form additional NOX and also the formation of nitrous oxide is a known issue:4NH3+5NO+3O2→4N2O+6H2O  (Reaction 4)4NH3+5O2→4NO+6H2O  (Reaction 5)
Besides nitrogenous containing compounds other compounds can also be used as reducing agents in the SCR reaction of NOx. Especially the use of hydrocarbons (HC) can also be used to selectively reduce nitrogen oxides (HC-SCR).
A general issue in the abatement of NOX from exhaust or flue gas systems from internal combustion engines, power plants, gas turbines, gas engines and the like is the penalty in pressure drop when a catalytic converter or any other article is introduced into the exhaust or flue gas system. The penalty arises because of the additional pressure required to push the exhaust or flue gas through the catalytic converter. Any decrease in the pressure drop over the catalytic converter will have a positive influence on efficiency and economy of the process. One method to decrease the pressure drop is by decreasing the size of the catalytic converter without compromising the NOX reduction efficiency, which requires the use of a more active catalyst composition. Therefore, any increase in catalyst activity is warranted.
Aluminosilicate zeolites and silicoaluminophosphate zeotypes are used as catalyst for SCR of NOX. For NH3-SCR the zeolite is typically promoted with transition metals. The most common used transition metals are iron and copper and the most commonly tested zeolite frameworks are *BEA, MFI and CHA (all given by the three-letter code devised by the International Zeolite Association).
Zeolite-based catalysts offer an alternative to vanadium-based SCR catalysts. Promoted with copper, zeolites typically exhibit a higher activity for NH3-SCR than vanadium-based catalyst at low temperatures (e.g. <250° C.) and upon high-temperature excursions toxic volatile compounds are not released upon catalyst degradation, which can be the case for vanadium-based catalysts. One limitation of the use of Cu-zeolites is that they do not provide a high NH3-SCR selectivity at high operational temperatures, approximately above 350° C. Iron-promoted zeolites on the other hand offer a high selectivity towards NH3-SCR at temperatures above 350° C. at the expense of high activity at lower temperatures (e.g. around 150-200° C.).
Since all combustion processes lead to water being present in the exhaust or flue gas, there is a requirement for a high hydrothermal stability of the NH3-SCR catalyst situated in a system wherefrom NOX should be removed. Especially the presence of water in the exhaust or flue gas is detrimental for zeolite-based catalysts since they are known to deactivate due to hydrolysis or degradation of the framework in presence of steam. Without being bound by any theory we believe this is related to dealumination of the aluminosilicate zeolite and thus will depend on the specific zeolite framework topology as well as the presence and identity of any extra-framework species hosted inside and onto the zeolite.
In general, there are several issues related to the use of metal promoted zeolites as SCR catalysts. First of all, the hydrothermal stability of the zeolite is not always sufficient. Since there will typically be some amount of water present, this, will in combination with high-temperature excursions, lead to dealumination and collapse of the crystalline microporous structure of the zeolite, that will ultimately lead to deactivation of the catalytically active material. Secondly, any hydrocarbons present will adsorb and deactivate the zeolite catalyst. Additionally, the presence of sulfur containing species (e.g. SO2 and SO3 etc.) in the system will lead to deactivation of the zeolite catalyst. In addition, formation of unwanted N2O also occurs. Furthermore, unwanted oxidation of ammonia at higher temperatures also occurs.
In terms of the transition metal introduced into the zeolite it is generally accepted that Cu-promotion leads to a higher NH3-SCR activity (see Reaction 1-3) at low temperatures (<300° C.) compared to Fe. However, Cu-promoted materials also produce more N2O (Reaction 4) and are less selective for the NH3-SCR reaction at higher temperatures (>300° C.) due to unselective ammonia oxidation (Reaction 5). When it comes to the influence of the transition metal the hydrothermal stability seems to be more dependent on the specific type of zeolite and zeotype framework. For example, Fe-*BEA materials are typically more hydrothermally stable than Cu-*BEA materials, whereas Cu-CHA materials are more hydrothermally stable than Fe-CHA materials [F. Gao, Y. Wang, M. Kollár, N. M. Washton, J. Szanyi, C. H. F. Peden, Catal. Today 2015, 1-12]. It is also generally accepted that Fe-promoted materials produce less N2O than their Fe-based equivalents [S. Brandenberger, O. Kröcher, A. Tissler, R. Althoff, Catal. Rev. 2008, 50, 492-531].
In the last years, it has been described that copper-containing small-pore aluminosilicate and silicoaluminophosphate Cu-CHA materials, Cu-SSZ-13 and Cu-SAPO-34 respectively, show high catalytic activity and hydrothermal stability for use as NH3-SCR catalyst [U.S. Pat. No. 7,601,662 B2; European Patent 2150328 B1, U.S. Pat. No. 7,883,678 B2].
[F. Gao, Y. Wang, N. M. Washton, M. Kollar, J. Szanyi, C. H. F. Peden, ACS Catal. 2015, DOI 10.1021/acscatal.5b01621] investigate the effect of alkaline and alkaline co-cations in Cu-CHA aluminosilicate SSZ-13. They find that certain co-cations in combination with the promotor metal-ion can enhance the activity as well as the hydrothermal stability of the Cu-CHA-based material. The study is however, limited to aluminosilicate zeolite SSZ-13 (CHA-zeolite) and any conclusions based on this material cannot be transferred to other aluminosilicate zeolite materials, frameworks or other promotor metal based zeolite systems.
Another zeolite topology related to that of CHA is the AEI topology. This structure also exhibits small pores (defined by eight oxygen atoms in micropore windows of the structure), similar to the CHA structure. Thus, without being bound by any theory, some of the benefits from using a CHA zeolite or zeotypes should also be present in the use of AEI based zeolite and zeotype. A method of synthesis of aluminosilicate AEI zeolite SSZ-39 was first disclosed in U.S. Pat. No. 5,958,370 using a variety of cyclic and polycyclic quaternary ammonium cation templating agents. U.S. Pat. No. 5,958,370 also claims a process for the process for the reduction of oxides of nitrogen contained in a gas stream in the presence of oxygen wherein said zeolite contains metal or metal ions capable of catalyzing the reduction of the oxides of nitrogen.
U.S. Pat. No. 9,044,744 B2 discloses an AEI catalyst promoted with about one to five weight percent of a promoter metal present. U.S. Pat. No. 9,044,744 B2 is ambiguous about the content of alkali and alkaline earth metals in the zeolite. In the description of U.S. Pat. No. 9,044,744 B2 a certain embodiment is mentioned where the catalyst composition comprises at least one promoter metal and at least one alkali or alkaline earth metal. In another embodiment the catalyst is essentially free of any alkali or alkaline earth metals except potassium and or calcium. However, there is no discussion or mention of the benefits of alkali or alkaline earth metals being present in the catalyst.
U.S. Patent 20150118134 A1 and [M. Moliner, C. Franch, E. Palomares, M. Grill, A. Corma, Chem. Commun. 2012, 48, 8264-6] teaches us that the AEI zeolite framework promoted with copper ions is a stable zeolite NH3-SCR catalyst system for treating the exhaust gas from an internal combustion engine. The Cu-AEI zeolite and zeotype catalytic system is stable during regeneration of an up-stream particulate filter up to 850° C. and water vapour content up to 100%. However, the effect of alkali is not discussed. Furthermore, the patent applications is solely concerned about the use of copper as a promoter metal ion, and the effect can therefore not be transferred to catalytic systems with other promoter metal ions.
WO 2015/084834 patent application claims a composition comprising a synthetic zeolite having the AEI structure and an in situ transition metal dispersed within the cavities and channels of the zeolite. In situ transition metal refers to a non-framework transition metal incorporated into the zeolite during its synthesis and is described as a transition metal-amine complex.
The use of Cu-amine complexes has been extensively described in the last years for the direct synthesis of Cu-containing zeolites, especially Cu-CHA materials [L. Ren, L. Zhu, C. Yang, Y. Chen, Q. Sun, H. Zhang, C. Li, F. Nawaz, X. Meng, F.-S. Xiao, Chem. Commun. 2011, 47, 9789; R. Martinez-Franco, M. Moliner, J. R. Thogersen, A. Corma, ChemCatChem 2013, 5, 3316-3323.; R. Martinez-Franco, M. Moliner, C. Franch, A. Kustov, A. Corma, Appl. Catal. B Environ. 2012, 127, 273-280; R. Martinez-Franco, M. Moliner, P. Concepcion, J. R. Thogersen, A. Corma, J. Catal. 2014, 314, 73-82] and lately also for Cu-AEI materials [R. Martinez-Franco, M. Moliner, A. Corma, J. Catal. 2014, 319, 36-43]. In all cases, the transition metal is stabilized by complexing with a polyamine. However, no report exists on the direct synthesis of Fe-AEI zeolites wherein the promotor metal is iron and where the iron does not require a complexing agent such as polyamine.
In many applications it is beneficial to have a high catalytic activity at temperatures >300° C. and at the same time have a high selectivity towards the NH3-SCR reaction (Reaction 1-3) without forming nitrous oxide or unselective ammonia oxidation (Reaction 4-5). In such applications iron-promoted zeolites are preferred.
Another benefit of zeolite catalysts is that in some cases they may be able to decompose nitrous oxide at higher temperatures [Y. Li, J. N. Armor, Appl. Catal. B Environ. 1992, 1, L21-L29]. Fe-*BEA zeolites are in general highly active in this reaction [B. Chen, N. Liu, X. Liu, R. Zhang, Y. Li, Y. Li, X. Sun, Catal. Today 2011, 175, 245-255] and should be considered state-of-the-art.
In applications where the catalyst is exposed to high temperatures it is also necessary to maintain the catalytic activity without severe deactivation. Typically, the gas stream wherein the catalyst will be situated contains some amount of water. For this reason, the hydrothermal stability of the catalyst should be high. This is especially detrimental for zeolite-based catalyst as they are known to deactivate due to hydrolysis or degradation of the framework in the presence of steam.
Some Cu-promoted zeolites exhibit a high hydrothermal stability and can typically tolerate temperature excursion up to about 850° C. However, this is not the case for Fe-promoted zeolites and the hydrothermal stability of Fe-promoted zeolites is in general lower than Cu-zeolites. The fact that Fe- and Cu-zeolites deactivate in a different manner is further corroborated in a study by Vennestrom et al. [P. N. R. Vennestrom, T. V. W. Janssens, A. Kustov, M. Grill, A. Puig-Molina, L. F. Lundegaard, R. R. Tiruvalam, P. Concepcion, A. Corma, J. Catal. 2014, 309, 477-490].
We have found that when decreasing the alkali metal content in iron promoted AEI zeolites, the hydrothermal stability is increased. By decreasing the alkali content, which is naturally present after synthesis of AEI zeolites, the stability of iron-promoted AEI zeolite becomes higher than other zeolite systems with similar iron contents. The zeolite catalyst of the present invention provides improved hydrothermal stability, high selectivity towards selective catalytic reduction at temperatures above 300° C. and low selectivity towards unselective ammonia oxidation and formation of nitrous oxide.